The invention relates to a constant velocity fixed joint with the following features: an outer joint part forming an annular member with a first longitudinal axis and comprising first ball tracks; an inner joint part forming a hub with a second longitudinal axis and comprises second ball tracks; first and second ball tracks associated with one another in pairs; and an annular ball cage arranged between the outer joint part and the inner joint part and comprising circumferentially distributed cage windows in which torque transmitting balls are held in a common plane.
The pairs of ball tracks of fixed joints are normally positioned in radial planes and are designed in such a way that the center lines of the first ball tracks and of the second ball tracks behave mirror-symmetrically relative to one another with respect to a central joint plane and intersect one another in the central joint plane. Tangents on the ball tracks in the central plane form oppositely directed control angles of identical sizes with the respective longitudinal axes of the two joint parts.
The production of ball tracks of the above design which are normally curved and which, if viewed in the axial direction, are sometimes undercut, is disadvantageous from the point of view of production technology.
From DE 42 28 230 A1 there are known joints with pairs of tracks whose center lines extend at a distance from one another at an angle of crossing relative to the respective center lines and which intersect one another in pairs. Said joints are axially fixed as a result of the mutual surface contact mutual engagement between spherical areas on the inner face of the outer joint part and the outer face of the ball cage or also between spherical areas on the outer face of the inner joint part and the inner face of the ball cage. When the joint rotates in an articulated condition, said mutual surface contacts generate high friction and thus disadvantageously high operating temperatures in the joint. In the case of axial loads, the surface parts contacting one another may be subject to self-inhibition and wedging.
It is an object of the present invention to provide constant velocity fixed joints which can be produced economically and which, at least within the range of small articulation angles, are suitable for high speeds.
According to the present invention, the foregoing object is achieved by a constant velocity fixed joint comprising an outer joint part forming an annular member with a first longitudinal axis and comprising first ball tracks. The first ball tracks extend at a distance from the first longitudinal axis and each form a first angle of crossing therewith. The joint also includes an inner joint part forming a hub with a second longitudinal axis and comprising second ball tracks. The second ball tracks extend at a distance from the second longitudinal axis and each form a second angle of crossing therewith. The first and second ball tracks are circumferentially distributed and associated with one another in pairs, with the first angles of crossing and the second angles of crossing of the pairs of ball tracks being identical in size and opening in opposite directions relative to the longitudinal axes. When the longitudinal axes coincide, the points of intersection of the pairs of ball tracks form a central joint plane. The joint further includes an annular ball cage arranged between the outer joint part and the inner joint part and comprising circumferentially distributed cage windows in which torque transmitting balls are held in a common plane.
In a first variant, the outer joint part comprises two separate circumferential areas which are positioned on either side of the central joint plane and which are interrupted by the first ball tracks and which, as inner guiding faces, are simultaneously in guiding contact with an outer counter face of the ball cage. In a second variant, the inner joint part comprises two separate circumferential areas which are positioned on either side of the central joint plane and which are interrupted by the second ball tracks and which, as outer guiding faces, are simultaneously in guiding contact with an inner counter face of the ball cage. In approximation, the ball cage forms a central portion of a spherical shell of an approximately constant wall thickness.
The present joint is advantageous in that joints of this type can be produced particularly easily and cost-effectively in that the tracks, especially at the outer joint parts, can be produced by simple machining operations, preferably by broaching. Subsequently, these tracks can be hardened. The undercut faces which are optionally required at the outer joint parts and by which the guiding faces are formed can be turned (hard-turned) after the ball tracks have been produced and hardened. The tracks may extend helically or in straight lines. In the former case, the angle of intersection has to be referred to tangents to the tracks.
By reducing the guiding faces to two annular faces interrupted by tracks, the inner friction is low and the running-in phase takes place at an accelerated rate. Even during the running-in phase, the heat development remains uncritical. Corresponding contact pairing takes place at an early stage, which results in a good rate of efficiency.
It is particularly advantageous if, in the first variant, the outer joint part and, in the second variant, the inner joint part is permanently provided with play relative to the respective counter face of the ball cage. As a result, the forces acting on the ball cage are reduced, and in this way, the development of heat is even easier to control, with a greater efficiency being achieved. However, in principle, it is also possible to provide cage guiding faces both at the outer joint part and at the inner joint part.
According to the first variant, the outer joint part is provided with inner guiding faces. According to the above, it is preferably the outer face of the inner joint part which is provided with play relative to the inner surface of the ball cage. Said outer face of the inner joint part can be formed, for example, by a smaller spherical face with a central cylindrical turned region. In this, the relatively large effective diameter of the guiding faces is advantageous in that it keeps the area pressure low.
The sub-claims and figures describe some examples for designing the guiding faces at the outer joint part and for designing the respective counter faces to which reference is hereby made. The two surfaces which contact one another can differ from said examples in that they deviate from a spherical form.
According to the second variant, the guiding faces are provided on the outside of the inner joint part, with the respective counter faces being provided on the inside of the ball cage. Again, according to the above, it is advantageous if the inner face of the outer joint part is provided with play relative to the outer face of the ball cage, the first being preferably entirely cylindrical because, from a production point of view, this is particularly easy. This is advantageous in that undercut faces at the outer joint part can be eliminated, which simplifies production.
For these designs of guiding faces and corresponding counter faces, too, it is possible to find examples in the sub-claims and figures to which, again, reference is made. In this case, too, the two surfaces which contact one another can differ from the examples mentioned in that they deviate from a spherical form.
As a result of providing the ball cage with play relative to the respective second one of the joint parts, functioning is not adversely affected, not even in those cases where, when the joint is articulated, the shape of the guiding faces forcibly causes the ball cage to be offset or displaced relative to the joint center. This is always the case if the sliding surfaces at the cage are not provided in the form of entirely (with reference to the joint center) centric spherical faces.
Especially by using existing equipment normally used for VL plunging joints, the joints in accordance with the invention can be produced advantageously. At smaller to medium-sized angles, they are functionally safe, even at high speeds. They are therefore highly suitable as fixed joints in unsteered rear axles of motor vehicles where they can be combined with prior art cross-groove joints in driveshafts. A particular advantage refers to the fact that they are extremely short in the axial direction. This does not mean that the outer joint part, which is basically annular, cannot be provided with an integrally formed-on base. Due to the fact that the balls are always controlled by the angles of intersection of the tracks and that, in consequence, the ball cage has no active control function, it is possible to fit the balls in the ball tracks in a radially play-free way, i.e. the balls are subjected to pressure in the pairs of tracks. As a result, the joint operates at a low noise level.